Lithography is a key factor in the drive for higher levels of microcircuit integration. Dynamic RAMs have quadrupled in the level of integration every three years as a result of the reduction in minimum geometries and increases in chip size. As minimum geometries approach 0.5 .mu.m and below, lithography alternatives include optics, electron beam direct write, X-ray and electron/ion beam proximity technologies. The latter three technologies are still in their infancy relative to optical lithography and still have obstacles to overcome, including decreased throughput, low source brightness and mask complexity, respectively.
While optical lithography continues to be the dominant technology because it is well established and is capable of implementing sub-micron resolution at least as low as 0.35 .mu.m, efforts into attaining smaller geometries are looking toward the newer technologies. In both optical lithography and its alternatives, progress into the realm of shorter wavelengths introduces increased sensitivities to minute surface imperfections including contaminants on optical surfaces, aberrations introduced by lenses and mirrors, photoresist thickness and surface variations and wafer flatness and planarity.
Addressing the issue of aberrations, due to manufacturing limitations most optical elements have plane, spherical or paraboloidal form. The restriction to surfaces of simple form imposes limitations in an optical system's ability to realize diffraction-limited performance due to aberrations, especially where a larger field of view is desired, as in a stepper. Aspheric surfaces can provide correction of the aberrations at the cost of substantially increased manufacturing complexity. By making one surface of a centered optical system aspherical it is possible to achieve exact axial stigmatism. By the use of two aspheric surfaces, the system may be made aplanatic, facilitating attainment of a diffraction-limited beam. Production of a conventional aspheric requires the removal of material from a surface by an iterative cycle of polishing then measuring the surface to achieve the asphere. This technique is severely limited by the lack of precision in the measurement techniques.
Where the illuminating light is coherent and has a narrow bandwidth, a binary phase plate, which is usually a thin plate of transparent material with steps of .lambda./2 formed on its surface, will act as an asphere. As with aspheres, the use of two phase plates will provide the desired aspheric shapes to make the optical system diffraction-limited. A reflective surface may similarly be made aspheric by the use of phase steps. More than two values, i.e., binary, of the phase modulation can be used in order to avoid scattering the edges of large steps.
The manufacturing process for phase plates is substantially easier than that for conventional aspheres, using common lithographic techniques. Since most steppers use narrowband illumination it would be desirable to provide a stepper illumination system in which aspherization is accomplished by phase plates, thereby providing a large field of view in a diffraction-limited system. It is to such a system that the present invention is directed.